1. Field of the Invention
The present invention relates to a voltage controlled variable capacitor which capacitance value is changed in accordance with a control voltage applied thereto.
2. Description of the Related Art
In signal processing circuits used in communication terminals etc., oscillators oscillating with high precision are desired in order to generate clock signals accurately. Conventionally, in order to improve the oscillation accuracy of a crystal oscillator used in general, there is known a VCXO (voltage controlled crystal oscillator) that includes a voltage controlled variable capacitor (a voltage controlled varactor) which capacitance value changes in accordance with a control voltage and a crystal vibrator which oscillates at a frequency according to the capacitance value of the voltage controlled varactor. There is also known a TCXO (a temperature compensated crystal oscillator or a crystal oscillator with a temperature compensation circuit) in which, in order to compensate the frequency variation of a crystal vibrator depending on a temperature, a control voltage which is compensated in temperature characteristics by a temperature compensation circuit is applied to a voltage controlled varactor thereby to oscillate the crystal vibrator at a desired frequency.
As a conventional voltage controlled varactor used in the aforesaid oscillator, there is one shown in FIGS. 12 to 14.
FIG. 12 shows a voltage controlled varactor using a diode. The voltage controlled varactor is arranged in a manner that the anode side of a PN junction diode is grounded and a terminal Zc on the cathode side thereof is applied with a control voltage, whereby the width of a depletion layer formed at the pn junction surface of the diode is changed thereby to change a capacitance value seen from the terminal Zc. The voltage controlled varactor shown in the figure configured to apply a reverse bias to the diode can change the capacitance value thereof by changing the control voltage in a certain range. However, in general, such the voltage controlled varactor can not make so high the sensitivity of the change of the capacitance value with respect to the change of the control voltage.
FIG. 13 is a voltage controlled varactor using a fixed capacitor and a MOS transistor. The voltage controlled varactor is arranged in a manner that the one end of the fixed capacitor and the drain of the MOS transistor are connected in series and the source of the MOS transistor is grounded, whereby the gate voltage of the MOS transistor is changed to thereby change the on-resistance value of the MOS transistor to change a capacitance value seen from the other end Zc of the capacitor (see Japanese Patent No. 3222366, for example).
FIG. 15(A) is a diagram showing characteristics (C-V characteristics) of the change of a capacitance value seen from the terminal Zc with respect to the control voltage Vcont of the voltage controlled varactor shown in FIG. 13. When the control voltage Vcont applied to the gate is gradually increased from the ground voltage and exceeds a threshold value Vt, the MOS transistor starts to turn on. That is, the on-resistance value of the MOS transistor decreases as the control voltage Vcont increases. Thus, the capacitance value of the voltage controlled varactor rapidly changes from 0 farad (F) to C (F) within a controlled voltage range ΔVC as shown in FIG. 15(A). As shown in this figure, this C-V characteristics is not good in linearity.
FIG. 15(B) is a diagram showing the changing rate (C-V sensitivity) of the capacitance value of the voltage controlled varactor with respect to the control voltage Vcont. As shown in this figure, the C-V sensitivity has no range representing almost the same value within the controlled voltage range VC. In other word, it will be understood that the changing characteristics of the capacitance value of the voltage controlled varactor is not linear.
FIG. 15(C) is a diagram showing the characteristics (f-V characteristics) of the oscillation frequency with respect to the control voltage Vcont in the case of using the voltage controlled varactor in a voltage controlled crystal oscillator. As shown in this figure, when the control voltage Vcont is changed in the voltage range ΔVC by using the voltage controlled varactor, the capacitance value of the voltage controlled varactor changes rapidly and further the changing characteristics is not linear. Thus, the oscillation frequency of the voltage controlled crystal oscillator can not be changed linearly.
That is, according to the voltage controlled varactor thus configured, although the sensitivity of the change of the capacitance value with respect to the change of the control voltage is high, the control voltage can be changed only in a small voltage range. Further, the linearity of the change of the capacitance value with respect to the change of the control voltage is not good. Thus, the voltage controlled varactor has difficulty in its control and hence has difficulty in using for compensating the frequency of the crystal oscillator.
FIG. 14 is a voltage controlled varactor in which the aforesaid conventional technique has been improved. This voltage controlled varactor is arranged in a manner that a plurality of the varactors each shown in FIG. 13 are connected in parallel, and control voltages Vcont1 to Vcontn having different values (Vcont1>Vcont2> - - - >Vcontn) are applied to the gates of MOS transistos (M1 to Mn) constituting the varactors, respectively, thereby to change the capacitance value seen from the terminal Zc. The respective control voltages are generated so as to have a predetermined deviation value therebetween in a manner that Vcont1=Vc, Vcont2=Vc−Voff1, Vcont3=Vc−Voff2 - - - . These control voltages are applied to the MOS transistors, respectively. Thus, when the voltage Vc is raised from the GND voltage and when Vcont1 (=Vc) reaches a threshold voltage VT at which the MOS transistor starts to turn on, the capacitance value of a varactor VC1 including the MOS transistor M1 starts to change. When the voltage Vc is further raised and reaches VT+Voff1 (that is, Vcont2=Vt), the capacitance value of a varactor VC2 including the MOS transistor M2 starts to change. In this respect, the value of Voff1 is determined in a manner that when the changing value of the capacitance value of the varactor VC1 becomes smaller after the capacitance value thereof starts to change, the capacitance value of the varactor VC2 starts to change. In the similar manner, the values of Voff2, Voff3 - - - are determined.
According to such a voltage controlled varactor, the control voltages having the different values are applied to the varactors, respectively. When the voltage Vc is increased, at first, the capacitance value of the varactor VC1 starts to change, and when the control voltage increases by the predetermined value (Voff1), the capacitance value of the varactor VC2 starts to change. Thus, the capacitance value (the entire capacitance value of the voltage controlled varactor) seen from the terminal Zc can be changed in a wide controlled voltage range. Further, according to this voltage controlled varactor, as described above, since the control voltages (Vcont1 to Vcontn) are applied in the predetermined pattern to the respective MOS transistors, the capacitance value seen from the terminal Zc can be changed linearly (see JP-A-10-51238, for example).
However, according to the aforesaid conventional voltage controlled varactor in which a plural sets of the fixed capacitor and the MOS transistor are connected in parallel, since the control voltages respectively having different values are applied to the MOS transistors, respectively, a plurality of circuits each for generating the control voltage are required. Thus, the circuit configuration becomes complicated and it is difficult to control the capacitance value with a high precision. Further, since the circuit configuration becomes complicated, it is difficult to reduce the circuit scale thereby to reduce a chip area.